Conventional frames have been assembled by joining frame members having connection surfaces cut to 45.degree. at both sides to be assembled rectangular each other, attaching L-shaped corner connection fittings to the connecting parts, fixing them to the frame members by rivets or set screws, and assembling frame members to a square frame.
Therefore, conventional frames have disadvantages in that they are composed of many components, they increase assembly work time and they create a troublesome job. This not only increases the production cost but also their appearance is not satisfactory with corner fittings being exposed on the outside. Further, conventional frames have been fixing the connection part with a corner connection fixture, therefore, it was necessary that the fixture be removed each time the picture is to be changed, resulting in substantial re-assembly time and troublesome work. There are also many accompanying disadvantages in that the picture replacing work is performed from the back side and the location of the picture is apt to be shifted, which requires much time for adjusting.